Such an engine commonly utilizes cam driven poppet valve to control inlet and exhaust of gases to and from the combustion chamber of the or each cylinder of the engine. Typically the timing of valve events in such an engine is fixed and hence a compromise, although the cam-driven poppet valve has the advantage that valve events can be varied over a wide range during design by varying the profile of the operating cam. Thus the timing and velocity of opening and closing and the dwell of the valve in its open position can be determined at the design stage. It is possible to provide for variable valve events in a popper valves system utilizing more or less complex drive mechanisms, and to provide for relatively rapid opening and closing of the valves, but the head and stem of poppet valve mechanism is of such a configuration as to stand in the way of optimally streamlined inlet and exhaust passages, and in practical embodiments protrudes when open into the combustion chamber of the cylinder which it controls. This may place design constraints on possible valve events if catastrophic failure through valve to piston contact is to be avoided. The reciprocating mass of the valve and the usual reliance upon springs to close place further constraints upon high speed operation if valves bounce or chatter is to be avoided.
Proposals have been made for rotary cylindrical or spherical valves, as exemplified by U.S. Pat. No. 5,361,739 and other earlier patents of George J. Coates. Such valves are driven by suitable gearing from the crankshaft of an engine of which they form part, and have the potential advantages of avoiding the use of reciprocating parts in the valve mechanism and permitting the provision of optimally shaped gas passage through the valve. In practice, however, the constant relationship of the rotational velocity of such valves to that of the engine they control places considerable constraints on design since the opening and closing velocity of the valve is constant, dwell in the fully open position can only be achieved by less than optimal valve passage shaping, and the shaping of the valve passage changes continuously while the valve is open. Little can be done to offset the relatively slow opening and closing of such a valve without seriously compromising valve passage design.